Since salt cod (bacalhau) is always sold salted and dried, it's necessary to desalt and rehydrate it before it can be used in cooking. Depending on how salty the fish is, this process should begin 24 to 48 hours prior to cooking the fish. If the bacalhau is very dry and covered with salt, start earlier. If it still has some moisture in the flesh and no visible salt, you can begin only 24 hours before.
Prior to the desalting and rehydrating process, if the bacalhau is still in the form of fillets, the fillets should be cut into thick slices of approximately 3 inches each. Most bacalhau is sold with the skin of the fish still attached, and it's better (and easier) to remove the skin prior to desalting. Simply lift the skin away from the flesh with a sharp knife at one corner of the piece, and then grasp the skin with a hand towel and firmly peel it away from the flesh.
Place the skinless pieces of bacalhau in a large bowl, preferably of glass or ceramic, then cover with cold, fresh water. The bowl should then be covered and placed in the refrigerator. (Bacalhau desalted at room temperature spoils very quickly). Every 6 hours or so, the fish must be removed from the refrigerator, drained, rinsed, returned to the bowl and covered with fresh water.
You can separate a small sample of fish and taste it to determine if it has been sufficiently desalted prior to begin cooking. The sample does not have to be cooked - bacalhau can be eaten safely uncooked, as the preserving technique "cooks" the flesh without heat.